Polypropylene has been used in great quantities as a resin having excellent rigidity, heat resistance, electric properties and chemical stability and high molding property.
However, polypropylene has the severe fault that injection moldings thereof have very poor dimensional accuracy to cause sink or warp, as it is a crystalline resin. Consequently, polypropylene has hardly been used in uses requiring high dimensional accuracy, such as precision parts, business instruments, domestic electric appliances, etc.
To improve the dimensional accuracy of polypropylene, the addition of inorganic fillers is effective, and plate or fibrous fillers are particularly effective.
However, when a fibrous filler, for example, glass fibers, is added, injection moldings have anisotropy in the coefficient of molding contraction in the molding direction (lengthwise direction) and in the rectangular direction (cross direction). Consequently, such moldings cannot be used for precision parts or goods requiring a good external appearance because high warpage is caused.
Further, among the plate fillers it is known that talc is excellent from the viewpoint of the rigidity and impact resistance of the polypropylene compositions.
However, prior art polypropylene compositions to which talc has been added have poor dimensional accuracy and cause sink or warp because of their high coefficient of molding contraction. Therefore, they cannot be put to practical use as molding materials.
It has also been proposed to add talc and natural rubber in order to reduce the coefficient of molding contraction (for example, see Japanese Patent Application (OPI) No. 136735/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")). However, even following this procedure molding contraction is so great it causes sink or warp, and such compositions cannot be practically used for precision parts or goods requiring a good external appearance.